This invention relates to a pressurized fluidized bed combustion system and method and, more particularly, to such a system incorporating a integral heat exchanger for recycling solids from the combustor.
According to prior art fluidized bed combustion systems and methods, air is passed through a bed of particulate material, including a fossil fuel, such as coal, and a sorbent for the oxides of sulfur generated as a result of combustion of the coal, to fluidize the bed and to promote the combustion at a relatively low temperature. These types of systems are often used in steam generators in which water is passed in a heat exchange relationship to the fluidized bed to generate steam and permit high combustion efficiency, fuel flexibility, high sulfur adsorption and low nitrogen oxides emissions. These types of systems often utilize a "circulating" fluidized bed in which the entrained solid particles of fuel and sorbent (hereinafter referred to as "solids") from the furnace are separated from the mixture of fluidizing air and combustion gases (hereinafter referred to as "flue gases") and are recycled back to the furnace.
In these circulating beds, the fluidized bed density is relatively low when compared to other types of fluidized beds, the fluidizing air velocity is relatively high, and the flue gases passing through the bed entrain a substantial amount of the fine solids to the extent that they are substantially saturated therewith.
The relative high solids recycling is achieved by disposing a cyclone separator at the furnace section outlet to receive the flue gases, and the solids entrained thereby, from the fluidized bed. The solids are separated from the flue gases in the separator and the flue gases are passed to a heat recovery area while the solids are recycled back to the furnace. This recycling improves the efficiency of the separator, and the resulting increase in the efficient use of sulfur adsorbent and fuel residence times reduces the adsorbent and fuel consumption. Also, the relatively high internal and external solids recycling makes the circulating bed relative insensitive to fuel heat release patterns, thus minimizing temperature variations and, therefore stabilizing the sulfur emissions at a low level.
When the circulating fluidized bed combustors are utilized in a steam generating system, the combustor is usually in the form of a conventional, water-cooled enclosure formed by a welded tube and membrane construction so that water and steam can be circulated through the wall tubes to remove heat from the combustor. However, in order to achieve optimum fuel burn-up and emissions control, additional heat must be removed from the system. This heat removal has been achieved in the past by several techniques. For example, the height of the furnace has been increased or heat exchange surfaces have been provided in the upper furnace to cool the entrained solids before they are removed from the furnace, separated from the flue gases and returned to the furnace. However these techniques are expensive and the heat exchange surfaces are wear-prone. Other techniques involve the deployment of an additional, separate heat exchanger between the outlet of the separator and the recycle inlet of the furnace. Although heat can be removed from the recycled solids in this separate heat exchanger before the solids are passed back into the furnace, these type of arrangements are not without problems. For example, it is difficult to precisely control the heat transfer rates in the recycle heat exchanger. Also, during startup or load low conditions, it is often difficult to bypass the heat exchange surfaces in the recycle heat exchanger. Further, in situations when the recycle heat exchanger is formed integrally with the furnace, there is often an increase in boiler plan area which adds to the cost of the system.